2. Field of the Invention
The present invention relates to improvement of a CMOS semiconductor device having an input protection circuit.
2. Description of the Related Art
Conventionally, two types of input protection circuits are known, one in which a diode is connected between an input terminal and a V.sub.DD line and a diode is connected between an input terminal and a V.sub.SS line; and the other in which a dummy transistor is connected between an input terminal and a V.sub.DD line and a dummy transistor is connected between an input terminal and a V.sub.SS line.
The first type of input protection circuit has the structure shown in FIG. 1. In FIG. 1, reference numeral 21 is an input terminal, T: a MOSFET, D.sub.1 : a diode of V.sub.DD side, D.sub.2 : a diode of V.sub.SS side, T.sub.1 : a P channel type MOSFET provided around the input protection circuit, and T.sub.2 : an N channel type MOSFET provided around the input protection circuit.
According to this type of the input protection circuit, if a plus surge V.sub.S+ (wherein V.sub.SS is used as a reference voltage, for example, V.sub.SS &lt;V.sub.DD &lt;V.sub.S+) is inputted to the input terminal 21, the plus surge V.sub.S+ passes through the diode D.sub.1 of the V.sub.DD side, and is absorbed in the V.sub.DD line. Moreover, if a minus surge V.sub.S- (wherein V.sub.SS is used as a reference voltage, for example, V.sub.S- &lt;V.sub.SS &lt;V.sub.DD) is inputted to the input terminal 21, the minus surge V.sub.S- passes through the diode D.sub.2 of the V.sub.SS side, and is absorbed in the V.sub.SS line.
The second conventional input protection circuit has the structure shown in FIG. 2. In FIG. 2, T.sub.3 is a dummy transistor of V.sub.DD side, and T.sub.4 is a dummy transistor of V.sub.SS side.
According to this type of input protection circuit, if the plus surge V.sub.S+ (wherein V.sub.SS is used as a reference voltage) is inputted to the input terminal 21, the plus surge V.sub.S+ passes through the dummy transistor T.sub.3 of the V.sub.DD side, and is absorbed in the V.sub.DD line. Moreover, if a minus surge V.sub.S- (wherein V.sub.SS is used as a reference voltage) is inputted to the input terminal 21, the plus surge V.sub.S- passes through the dummy transistor T.sub.4 of the V.sub.SS side, and is absorbed in the V.sub.SS line.
The first conventional input protection circuit, however, has the following disadvantage.
That is, if the pulse surge V.sub.S+ (V.sub.SS =reference voltage) is inputted to the input terminal 21, the plus surge V.sub.S+ is absorbed in the V.sub.DD line as shown by a solid line arrow. Thereafter, the plus surge V.sub.S+ passes through the V.sub.DD line and finally reaches a drain of the N channel type MOSFET T.sub.2. Then, the plus surge V.sub.S+ passes through a Zener diode comprising N.sup.+ and P.sup.-, and is absorbed in the V.sub.SS line. As a result, current caused by the surge flows in the drain of the N channel type MOSFET T.sub.2, and the drain is destroyed.
Similarly, in a case where the plus surge V.sub.S+ (V.sub.SS =reference voltage) is inputted to the V.sub.DD line, current caused by the surge flows in the drain of MOSFET T.sub.2, and the drain is destroyed.
If the minus surge V.sub.S- (V.sub.DD =reference voltage) is inputted to the input terminal 21, the minus surge V.sub.S- is absorbed in the V.sub.SS line as shown by a broken line arrow. Thereafter, the minus surge V.sub.S- passes through the V.sub.SS line and finally reaches to the drain of the N channel type MOSFET T.sub.2. Then, the minus surge V.sub.S- passes through the Zener diode comprising N.sup.+ and P.sup.-, and is absorbed in the V.sub.DD line. As a result, current caused by the surge flows in the drain of the N channel type MOSFET T.sub.2, and the drain is destroyed.
Similarly, in a case where the minus surge V.sub.S- (V.sub.DD =reference voltage) is inputted to the V.sub.SS line, current caused by the surge flows in the drain of MOSFET T.sub.2, and the drain is destroyed.
As mentioned above, according to the first conventional input protection circuit, MOSFET T can be protected. N channel MOSFET T.sub.2 provided around the MOSFET T, however, cannot be protected with respect to the plus surge V.sub.S+ (V.sub.SS =reference voltage) to be inputted to the input terminal 21 or the V.sub.DD line and the minus surge V.sub.S- (V.sub.DD =reference voltage) to be inputted to the input terminal 21.
As compared with the input protection circuit, the second conventional input protection circuit is improved in the following respect.
That is, if the plus surge V.sub.S+ (V.sub.SS =reference voltage) is inputted to the input terminal 21, as shown by an arrow a of a solid line, the plus surge V.sub.S+ finally reaches the drain of the N channel type MOSFET T.sub.2 and the drain of the dummy transistor T.sub.4 of the V.sub.SS side. Then, the plus surge V.sub.S+ passes through a Zener diode comprising N.sup.+ and P.sup.- of the drain sides of the transistors T.sub.2 and is T.sub.4, and absorbed in the V.sub.SS line. As a result, in the drains of the transistors T.sub.2 and T.sub.4, density of current caused by the surge is relaxed and the drains are prevented from being destroyed. However, in this case, since the dimension of the dummy transistor T.sub.4 of the V.sub.SS side must be made larger than that of the N channel type MOSFET T.sub.2, this is disadvantageous to the integration of the element.
If the plus surge V.sub.S+ (V.sub.SS =reference voltage) is inputted from the V.sub.DD line, the plus surge V.sub.S+ finally reaches the drain of the N channel type MOSFET T.sub.2 as shown in an arrow b of a solid line. Then, the plus surge V.sub.S+ passes through only Zener diode comprising N.sup.+ and P.sup.- of the drain side of the transistor T.sub.2, and is absorbed in the V.sub.SS line. Due to this, the drain of the N channel type MOSFET T.sub.2 is destroyed.
Moreover, if the minus surge V.sub.S- (V.sub.DD =reference voltage) is inputted to the input terminal 21, the minus surge V.sub.S- is absorbed in the V.sub.SS line as shown by an arrow of a broken line. Thereafter, the minus surge V.sub.S- passes through the V.sub.SS line and finally reaches to the drain of the N channel type MOSFET T.sub.2. Then, the minus surge V.sub.S- passes through the Zener diode comprising N.sup.+ and P.sup.-, and absorbed in the V.sub.DD line. As a result, current caused by the surge flows in the drain of the N channel type MOSFET T.sub.2, and the drain is destroyed.
Similarly, in a case where the minus surge V.sub.S- (V.sub.DD =reference voltage) is inputted to the V.sub.SS line, current caused by the surge flows in the drain of MOSFET T.sub.2, and the drain is destroyed.
As mentioned above, according to the second conventional input protection circuit, MOSFET T can be protected. An N channel MOSFET T.sub.2 provided around the MOSFET T, however, cannot be protected relating to particularly the minus surge V.sub.S -.